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<jats:title>ABSTRACT</jats:title><jats:p>The characterization of carbonate production on rift basins is critical for understanding the nucleation and demise of reefs in tectonically active areas. A new petrographic and mineralogical analysis of Cambrian strata from the Avalon Zone in Newfoundland, based on scanning electron microscopy – back‐scattered electron detector and Raman spectrometer analyses, facilitates recognition of several episodes of Terreneuvian–Miaolingian carbonate production and associated precipitation of ironstone and phosphorite. These distinct units mainly developed on uplifted rift shoulders and basaltic lava palaeoreliefs, and reflect amalgamated high‐energy events, interrupted by scouring discontinuities (diastems) commonly lined by phosphatized and ferruginized microbial crusts. Mud‐mounds, in contrast, nucleated under calm conditions episodically punctuated by high‐energy episodes, where scattered thromboid structures occur as both clotted textures and distinct calcimicrobes. Precipitation of hematite/goethite versus chamosite couplets, both occluding primary porosities and replacing interlaminae and cortices of oncoids and coated aggregates, point to marine substrates close to the Fe‐redox boundary. Upwelling of phosphate‐rich ferruginous hydrothermal waters contributed to the precipitation of ironstone and phosphate interbeds. Ferruginous waters related to penecontemporaneous hydrothermal activity, reflected by the record of synsedimentary fissuring and stockwork ore bodies, were delivered to confined rift‐related horst‐and‐graben settings, largely controlled by the development of specific Cambrian carbonate and associated ironstone facies. The influence of ferruginous waters necessarily affected the record of climatically sensitive evaporitic pseudomorphs, reefs/mounds and phosphorites, which are then not suitable criteria to discriminate palaeolatitude, as demonstrated by a comparison of low‐latitude to middle‐latitude margins fringing Baltica, and the Avalonian and Atlas – Ossa‐Morena – Northarmorican rift transects of West Gondwana.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Climate change poses a significant challenge for life on Earth. Different climate modes have been shown to come along with changes of the magnesium/calcium (Mg/Ca) ratio of seawater, and such changes are believed to control the primary mineral phase of marine authigenic carbonates. However, factors controlling marine carbonate phases other than seawater Mg/Ca ratios exist. Fibrous cements forming at methane seeps in the Black Sea provide new insight into the factors governing elemental and mineral phase compositions of fibrous carbonates. In this study, the distribution of aragonite and fibrous Mg calcite cements from three seep sites in the Black Sea is described as a function of water depth. The Mg/Ca ratio of seawater, as well as the ratio in shallow pore water, is close to four at the examined sites. Fibrous Mg calcite post‐dated aragonite cement in seep carbonates from shallow water depth of 120 to 190 m, whereas Mg calcite is the only cement at a greater depth of <jats:italic>ca</jats:italic> 2000 m. The primary formation of fibrous Mg calcite is confirmed by its zonation under cathodoluminescence, crystal morphologies agreeing with competitive growth, uniformly distributed MgCO<jats:sub>3</jats:sub> contents and precipitation in equilibrium with local conditions calculated from δ<jats:sup>18</jats:sup>O values. The MgCO<jats:sub>3</jats:sub> contents (4.5 to 12.2 mol%) are negatively correlated with δ<jats:sup>13</jats:sup>C values, indicating that the incorporation of Mg into the calcite crystal structure was favoured by high concentrations of sulphide generated by sulphate‐driven anaerobic oxidation of methane. Unlike open oceanic basins, stratification in the Black Sea leads to euxinic conditions in the deeper water column, favouring fibrous Mg calcite formation. This observation is consistent with sulphide catalysis as a critical agent for the formation of low‐Mg calcite to very high‐Mg calcite at high Mg/Ca ratios and is possibly relevant to carbonate cements forming during times of oceanic euxinia.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Autogenic feedbacks can produce large‐scale, organized stratigraphic patterns in alluvial fans, but autogenic depositional signatures of specific upstream boundary conditions remain challenging to interpret. Here, a combination of theory, experiment and field application is used to explore how autogenic lithofacies changes can be interpreted as stratigraphic indicators of upstream boundary conditions. Six experiments were conducted to test the effects of sediment supply and water discharge rates on autogenic advance and retreat of the lithofacies boundary (grain‐size transition) in an alluvial fan with two dominant grain sizes. Migration of the grain‐size transition caused a short‐term zigzag pattern in the grain‐size transition position in the dip‐directional deposit section. For each experiment, time‐lapse images and laser topographic scans of the fan surface and stratigraphic cross‐sections of the final deposits were used to quantify characteristic timescales of autogenic processes. Timescales for fan‐margin migration, surface wet‐fraction change and grain‐size transition migration generally shorten as sediment supply rate increases and water discharge rate decreases. Increasing the sediment supply rate shortens the duration of the fluvial sediment storage and release cycle, producing higher frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge tends to widen channels and lengthens the duration of the fluvial sediment storage and release cycle, constructing lower frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge also enables more sediment to transport further downstream during release events, leading to higher magnitude zigzags in the grain‐size transition trajectory. These relationships between upstream boundary conditions and the grain‐size transition trajectory demonstrate how autogenic stratigraphic signals could be used as a tool to infer relative changes in boundary conditions.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>The grain‐size distributions of sediments can yield important information about sediment provenance and sedimentary processes; however, grain‐size distributions are frequently polymodal, rendering analyses difficult. To improve analyses of polymodal grain‐size data, the present study decomposed the grain‐size distributions of bottom surface sediments from Lake Kitaura, a shallow lake in Japan, into lognormal distributions and performed logratio analysis of their mixing proportions. The polymodal grain‐size distributions of the studied samples were separated into four common components at most sites. This logratio analysis revealed clear differences in the characteristics of the spatial distributions of the separated grain‐size components. The logratio values indicated that the three finer components were uniformly deposited within the lake, whereas the coarsest component was spatially diverse, reflecting differences in their sources and sedimentary processes. These results demonstrate the effectiveness of decomposition and logratio analysis of polymodal grain‐size distributions for estimating sedimentary processes. This method can be applied to modern sediments and for palaeoenvironmental reconstructions using sediment cores.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Oxygen concentration in the ocean is vital for sustaining marine ecosystems. While the potential impacts of deoxygenation on modern oceans are hard to predict, lessons can be learned from better characterizing past geological intervals formed under a greenhouse climate. The greenhouse Cretaceous containing several oceanic anoxic events characterized by widespread oxygen‐deficient water is ideal in this regard. The Austin Chalk Group in south Texas (USA) shows organic‐rich intervals that can be linked to oxygen depletion in the ocean, but the exact bottom water oxygenation conditions have not been estimated. This study aims to reconstruct both sediment interstitial and bottom water oxygenation history during Austin Chalk Group deposition by integrating detailed ichnological, sedimentological and geochemical (X‐ray fluorescence and X‐ray diffraction) analyses, thereby providing a consistent model that may be applicable across a range of marine shelf settings. The 141.12 m Gise #1 core contains a continuous record of the Austin Chalk Group, providing an opportunity for unravelling oxygenation and deoxygenation events. Whereas the anaerobic–exaerobic deposits are essentially nonbioturbated, four oxygen‐related ichnocoenoses are defined, further refining the transition of aerobic to dysaerobic conditions in the sediment interstitial water. Omission surfaces and glauconitic grains, products of current‐induced scouring and condensation, suggest sporadic high‐energy events in the Austin Chalk Group ramp that drove elevated terrestrial inputs. Geochemical data further help to identify anoxic bottom water conditions within the anaerobic facies. Additionally, the lowermost part of the Austin Chalk Group illustrates redox cycles, whereas dilution events characterized by elevated terrestrial input are identified throughout the rest of the Austin Chalk Group. The evolution of oxygenation levels in sediment interstitial water and bottom water disputes the existence of a long‐lasting oxygen‐deficient sea in south Texas. The refined depositional model may be applicable to coeval shelfal settings. Moreover, the results provide insights into variable, evolving palaeoclimatic and palaeoceanographic conditions of the greenhouse Late Cretaceous.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Travertines are terrestrial carbonates that are commonly associated with fault activity in extensional and transtensional basins. The faults serve as conduits for the rising and mixing of carbonate‐enriched fluids with thermal and meteoric CO<jats:sub>2</jats:sub> inputs promoting travertine precipitation at the surface. Therefore, travertine successions provide key constraint on the faulting, depositional environments, fluid flow and climate. This work focuses on the travertine succession in the Miocene Levač Basin, the marginal basin of the Morava Corridor situated at the junction of the Dinarides and the southernmost Carpathians. Detailed sedimentological, geochronological (U–Pb age, laser ablation – inductively coupled plasma – mass spectrometry) and structural analyses of the travertines are used to reconstruct the evolution of the feeding geoothermal system. Furthermore, these data were used to understand the controlling factors governing alternation of fluid flows enriched in thermally generated and meteoric CO<jats:sub>2</jats:sub>, and precipitation of travertines in Levač Basin, and finally to elucidate the late stage of basin evolution. Four facies associations are distinguished within the succession, i.e. travertine slope, ridge, flat, and travertine flat under the fluvial influence. The results demonstrated that travertine deposition was controlled by north‐west/south‐west and north‐east/south‐east normal fault arrays. Stable isotope data show positive δ<jats:sup>13</jats:sup>C values (with δ<jats:sup>18</jats:sup>O being negative) shifting to negative in the distal and stratigraphically younger deposits implying dilution of deep hydrothermal fluids by mixing with meteoric waters. Finally, travertine deposits yielded a new U–Pb age of <jats:italic>ca</jats:italic> 14 Ma indicating that the Middle Miocene extensional phase known from other intermountain basins in the Dinarides reached as far east as the Levač Basin and Morava Corridor.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Strontium isotopes (<jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr) are commonly used to assess the age of Cenozoic dolostones found on many isolated oceanic islands. The possible influence of Rubidium (Rb) content on the <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr ratios, however, has generally been ignored. This critical issue is assessed by examining the <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr ratios and elements (Rb, Sr, Al, Ti and K) of seventy‐one samples that came from a <jats:italic>ca</jats:italic> 339 m thick cored interval of Cenozoic dolomite on the Xisha Islands, South China Sea. For the dolomite from the upper Pliocene Yinggehai Formation (180.5 to 308.5 m; average 23% dolomite) there is a positive correlation between the <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr ratios and Rb concentrations, whereas for the dolomite from the lower late Miocene Huangliu Formation (308.5 to 519 m; average &gt;98% dolomite) such a correlation is not evident. Combined with high‐temperature experiments, correlation between Al, Ti, K, Rb and <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr indicates that Rb containing old <jats:sup>87</jats:sup>Rb from terrestrial materials modified the <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr ratios of the Yinggehai dolomite, but not those from the Huangliu dolomite. This is mainly attributed to the different sources of terrestrial dust carried by different East Asian monsoon systems between the late Miocene and Pliocene, and probably also influenced by the dolomitization process to some extent. To test for <jats:sup>87</jats:sup>Rb contamination, extrapolation of results from the Yinggehai dolomite and Huangliu dolomite indicate that concentrations of Rb &lt; 1 ppm, Al &lt; 1000 ppm, Ti &lt; 15 ppm and K &lt; 250 ppm in the dolomite indicates that terrestrial inputs are minimal and that the measured <jats:sup>87</jats:sup>Sr/<jats:sup>86</jats:sup>Sr ratios can be used for determining the dolomitization age. Potentially, these concentration thresholds may serve as a screening criterion for assessing the reliability of Sr isotope chronostratigraphy of other island dolostones throughout the world.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>In contrast to the prevalence of dolomite [CaMg(CO<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub>] in the geological record, there are few instances of recent formation. This discrepancy occurs despite supersaturation with respect to dolomite in many modern marine and lacustrine environments. Additionally, laboratory experiments have struggled to precipitate dolomite at ambient temperatures (&lt;40°C) even under highly saturated conditions. However, recent work has found dissolved silica to be an effective catalyst for the direct precipitation of dolomite. To test this hypothesis, the hydrology and mineralogy of Deep Springs Lake, a playa setting with primary dolomite precipitation has been explored. In this study, the central playa sites with the highest sedimentation rates were found to have dissolved silica concentrations close to saturation. Rietveld refinement showed that bulk mineral assemblages of core samples contain 20 to 40% fine‐grained, partially ordered dolomite along with a 10 to 20% poorly crystalline clay fraction, while transmission electron microscopy imaging found a co‐precipitation relationship between the fine‐grained dolomite and Mg‐rich smectite clays, where the Mg‐rich smectite clays act as a sink for the excess dissolved silica during evaporation. Transmission electron microscopy results indicate that the nanodolomite crystals formed through surface‐induced nucleation and growth processes in the presence of dissolved silica as a catalyst. The broad coincidence of conditions for the direct precipitation of dolomite and Mg‐rich smectite clays provides evidence that silica may be a key to the dolomite problem, as well as constraining palaeoenvironments such as the Pre‐salt Barra Velha Formation.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>East Asia experienced complex climatic changes during the Eocene. Due to the lack of a high‐precision time framework covering the Eocene, the characteristics of sedimentary and climate evolution in this period remain unclear. The Jianghan Basin (Central China) developed an Eocene continental succession, which is composed of lower red beds and upper salt‐lake deposits exhibiting clear sedimentary rhythms, making these strata an ideal target for astronomical cycle analysis. Based on the established floating astronomical timescale, the lower red beds in the basin formed during the early–middle Eocene and were dominated by eccentricity cycles. The upper salt‐lake deposits formed during the middle–late Eocene and recorded obliquity cycles. The change in facies indicates that the climate transitioned from continuously arid to less arid with periodic relative wetness, which was forced by orbital cycles. Significant obliquity signals recorded by salt rhythmites responded to Eocene global temperature changes and Tibetan region uplift, which can be important markers of a monsoon‐like climate. Comparing the sedimentary and climatic characteristics of other Eocene lake basins in East Asia, the extension of the Eocene East Asian monsoon may have initiated during <jats:italic>ca</jats:italic> 43.6 to 40.0 Ma. This study provides a new perspective for understanding the early evolution of the East Asian monsoon.</jats:p>